Ink jet printers eject ink onto a print medium, such as paper, in controlled patterns of closely spaced dots. FIG. 1 is a schematic of a print system including a prior art multi-orifice ink jet print head array 10. To print dots on all major portions of a paper 16, print head array 10 is shuttled back and forth on guide rail 14 in the X direction, as shown in FIG. 1, as paper 16 is advanced in the Y direction. Edges 18A and 18B represent the respective left and right edges of paper 16.
Because it takes a finite time for an ink drop to travel from print head 10 to paper 16, print head 10 ejects an ink drop from a particular orifice before the orifice is aligned with the intended destination ("the target") of the drop. If an ink drop is ejected too early or too late, the ink drop will not strike the target. For print head 10 to eject the drop at the precise time, the controller of the print head must know the position of print head 10 with respect to paper 16.
An encoder scale 20, which is made of clear mylar base material, is suspended between supports 24A and 24B. Encoder scale 20 may also be made from more expensive materials such as thin glass or etched metal. Many regularly spaced regions 28 of opaque material are photographically printed along the length of encoder scale 20, as shown in FIG. 2. Encoder scale 20 is manufactured by Litchfield Precision Co.
Referring to FIG. 1, an encoder 34 is mounted to print head 10. Encoder 34 may be a model HEDS-9200, option P, encoder module marketed by Hewlett-Packard Corporation, Palo Alto, Calif. FIG. 3A shows a cross-sectional side view of the print system of FIG. 1, taken along lines 3--3. Referring to FIG. 3A, encoder 34 includes a light emitting diode (LED) 36, which emits collimated light toward a photodiode array 38. Referring to FIG. 3B, which is an enlarged front view of encoder 34, photodiode array 38 contains many photodiodes separated into two spatially distinct groups referred to herein as photodetector 40 and photodetector 42.
The outputs of the individual photodiodes in photodetector 40 are processed so that the output of photodetector 40 behaves as though photodetector 40 were a discrete photodetector of very small width in comparison with the width of one of opaque regions 28. The output of photodetector 42 similarly behaves as though photodetector 40 were a discrete photodetector of very small width in comparison with the width of one of opaque regions 28.
The outputs of the photodiodes in photodectors 40 and 42 are processed to produce respective nominally identical, 90.degree. phase-displaced SIN and COS signals. In the present invention, the COS signal is used only to determine the direction in which print head 10 is traveling.
FIG. 2 illustrates the voltage level of the SIN signal as encoder 34 is advanced along encoder scale 20. Referring to FIG. 2, opaque regions 28A, 28B, 28C, and 28D are spaced between clear regions 30A, 30B, 30C, 30D, and 30E. There are typically 150 of opaque regions 28 per inch (150 lpi) (5.9 opaque regions per millimeter (mm)) evenly spaced between clear regions 30. Accordingly, opaque regions 28 and clear regions 30 are each 0.0033 inches (0.0847 mm) wide. A first edge 44 of opaque region 28A is referred to as the positive edge, and a second edge 46 of opaque region 28A is referred to as the negative edge.
At time t0, photodetector 40 is positioned so that light from LED 36 is unimpeded by one of opaque regions 28 and, consequently, the SIN signal is a logic 0 ("low"). At about time t1, photodetector 40 moves past the positive edge of opaque region 28A. The voltage of the SIN signal rapidly increases to a logic 1 ("high").
A dot clock (not shown) provides output pulses at both the rising and falling edges of the SIN signal. Accordingly, the dot clock produces a pulse in response to the rising edge of the SIN signal at about time t1. At about time t2, photodetector 40 moves past the negative edge of opaque region 28A. Ideally, the state of the output of photodetector 40 would respond instantaneously to a change in detected light. However, stored charge and junction capacitance in photodetector 40 or some other reason causes the voltage of the SIN signal to decrease in response to a negative edge of an opaque region 28 much more slowly than the SIN signal increases in response to a positive edge. The dot clock does not produce a pulse in response to the negative edge until time t3. The relative time between times t2 and t3 is exaggerated to illustrate that the dot clock pulses are not evenly spaced apart. At about time t4, photodetector 40 traverses opaque region 28B. The voltage of the SIN signal rapidly increases to the high state, and the dot clock produces a pulse at about time t4. At about time t5, the SIN photodetector 40 moves past opaque region 28B; however, because of the slow response time of the SIN signal, the dot clock does not produce a pulse until about time t6.
The dot clock pulses indicate the position of print head 10 to the control system (not shown). As can be seen from FIG. 2, the dot clock pulses are not evenly spaced. (Errors in the spacing between adjacent dot clocks pulses are often much greater than .+-.7 percent.) Therefore, the control system will not always know the precise location of print head 10. For example, the control system will not know that print head 10 has passed opaque region 28A after a time delay following time t3. Depending on the standards of the precision of the particular printer, the time delay in the receipt of print head position information by the control system may be significant.
The difference in the rise and fall times of the SIN signal is not the only cause of lack of uniformity in dot clock pulse spacing. The problem may be enhanced by nonuniform sizes of opaque regions 28 printed on encoder scale 20.
One partial solution to the problem is to provide encoder scale 20 with a larger number of opaque regions, such as 300 opaque regions per inch (11.8 opaque regions per mm) rather than 150 opaque regions per inch (5.9 opaque regions per mm). An encoder capable of recognizing 300 opaque regions per inch (300 lpi encoder) could be used to produce an output pulse only in response to the detection of a positive edge. A major problem with this solution is that a 300 lpi encoder is much more expensive than an 150 lpi encoder, such as encoder 34. Moreover, a 300 lpi encoder is much more bulky, fragile, harder to apply, and less readily available than a 150 lpi encoder.
There is, therefore, a need for a method and system using relatively inexpensive components for accurately determining the position of a print head with respect to printing media.